Method of coating metals with a silicide layer and an outer layer of aluminasilicate



US. Cl. 11721 6 Claims ABSTRACT OF THE DISCLOSURE An article of niobium or other refractory metal alloy, having a mixture of metal powders such as iron, silicon, or chromium packed around it, is heated in an iodinehydrogen atmosphere whereby the metals become reacted together and become diffused into the surface of the article, thereby forming a dense coating, for example of niobium disilicide. In a further step, alumina or an alumina-silicate is deposited on the silicide coating.

The invention relates to coatings for refractory metallic materials.

Articles required to operate at elevated temperatures particularly when also under applied stress (e.g., rotor blades for gas turbines) for many years have been manufactured from nickel alloys. It has been found however that nickel alloys, generally, have inadequate strength for operation at temperatures much above 1000 C.

Consideration has been given to the use of more refractory alloys (such as those based on molybdenum, niobium, tantalum and tungsten) which maintain a useful level of strength above 1100 C. One drawback to the usefulness of these materials is their poor oxidation resistance at the temperatures involved.

The oxidation problem can be oifset by the use of protective coatings which must be both dense and adherent to the alloy.

Ceramic materials such as alumina have high resistance to oxidation but are not altogether suitable for coating refractory metals due to difliculties in obtaining a satisfactory bond coupled with a uniformly dense (i.e., nonporous) coating.

Applying a ceramic coating by flame spraying direct on to the metal surface is unsuitable because of the almost insoluble difliculty of obtaining a uniformly dense coating (i.e., without porosity). A fluidised bed reactiOn would give a dense coating but requires the presence of water vapour which would cause oxidation of the surface of the article to be coated in the process.

According to the invention there is provided a method of forming an overlay having high resistance to oxidation on a body of refractory metallic material including the step of bonding an intermediate coating of another material to the body.

According to a feature of the invention the intermediate coating is of a material which also has a high resistance to oxidation.

Preferably the overlay is of ceramic material such as alumina or an alumina-silicate and the refractory metallic material is a niobium alloy.

According to another feature of the invention, a composite coating having high resistance to oxidation and formed on a body of refractory metallic material comprises an intermediate layer of a silicide diffusion bonded to the body.

A specific example of the invention will now be described. In this example the basic article (i.e., substrate) nited States Patent 3,443,978 Patented May 13, 1969 is composed of the niobium alloy having the nominal composition W 15 percent, Mo 5 percent, Zr 1 percent and known as F48. This has a high temperature strength suitable for use in aircraft gas turbine rotor blading.

The substrate is packed in a mixture of silicon, iron and chromium powders of commercial purity in the proportions Si 58.54 percent, Fe 33.33 percent; Cr 8.13 percent. Variations in composition based on the siliconiron eutectic (i.e., 59 percent Si, 41 percent Fe) with chromium additions up to 15 percent have also given satisfactory results.

The pack is heated to 1100 C. in an iodine-hydrogen atmosphere and maintained under these conditions for five hours.

Iodides of the respective metals are formed but the presence of hydrogen causes reduction to the metallic state. The reversability of the process causes great activity whereby the metals dilfuse readily into the surface of the substrate to form a modified niobium disilicide coating.

After five hours the hydrogen supply is replaced by argon and the component diffusion heat treated for a further fifteen hours at 1100 C. before cooling.

The resultant coating is basically niobium disilicide, diffusion bonded to the substrate. It is quite dense appearing to be single phase (except at the interface), and quite clean, with a thickness of 0.002 to 0.003 inch.

In a further stage, the silicide coated article is pyrolised on a fluidised bed where it is subjected to a vapourised stream of steam and aluminum chloride to give an overlay of alumina by the reaction or the other alumina-silicate (such as one according to the molecular formula:

0.7 CaO 0.15 N320 0.7 A1203'=7-3 percent 0.15 K 0 These may be applied by the fluidised bed technique using a bed of silica or alternatively can be deposited in powder form by spraying or brushing and then heated to 1300" C. for five minutes to cause incipient fusion and bonding to the silicide coating.

A silicide coating, being ductile over a wide range of temperature and having a high resistance to oxidation, might appear to be a suitable protection in itself. It is true that silicide coatings potentially olfer the best protection at high temperatures in that they form a thin skin of silica glass on their outer surfaces during exposure to air or oxygen in such circumstances and as a rule this gives adequate protection in steady or slowly fluctuating working conditions.

However, in the case of a turbine rotor blade there may be a temperature gradient extending between about 600 C. at the root and 1300 C. at the tip and moreover it is subjected to frequent and rapid changes extending over a large part of this range (i.e., thermalcycling).

During thermal-cycling to intermediate temperatures silicide coatings are prone to pest failure in that cracks develop and extend through the coating. The resultant ingress of oxygen and nitrogen to the substrate causes it to be embrittled and possibly to disintegrate with consequent spalling of the coating.

The use of a ceramic overlay has been found to prevent or at least delay the formation of cracks.

The use of a silicide as an intermediate layer has however a distinct advantage in that protection of the substrate is maintained in the event of pinhole failure of the ceramic overlay. The combination of a silicide intermediate layer with a ceramic overlay thus aiiords twofold protection of the substrate.

The method is not necessarily limited to the use of a silicide as an intermediate coating-other suitable coatings may be obtained by the use of titanium, chromium, iron or aluminum either singly or in combination, the essential requirements being that the material selected will form a strong bond with the substrate and will not be subject to oxidation during the application of the overlay.

Nor is the method restricted to the protection of purely refractory metallic materials but may be applied to other metallic materials such as alloys of nickel, cobalt and chromium.

We claim:

1. A method of producing a metallic article having a substrate of a refractory alloy selected from the group comprising molybdenum, niobium, tantalum, tungsten, nickel, cobalt and chromium which comprises (1) packing said substrate in a mitxure of silicon, iron, and chromium powders, (2) heating in an iodine-hydrogen atmosphere to produce a silicide coating chemically bonded to the substrate and having a thickness in the range of about 0.002 to about 0.003 inch and (3) chemically bonding an ilumina-silicate to the silicide coating.

2. A method of producing a metallic article according to claim 1 wherein the alumina-silicate is deposited in powder form and subsequently heated to cause incipient fusion and bonding to the silicide coating.

3. A method of producing a metallic article according to claim 2 wherein said alumina-silicate is bentonite.

4. A method of producing a metallic article according to claim 3 wherein the proportions of silicon, iron, and chromium powders are based on the silicon-iron eutectic with chromium additions up to 15 percent.

5. A method of producing a metallic article according to claim 2 wherein said alumina-silicate is a material having the molecular formula:

0.7 CaO l 0.15 Na O 0.15 K 0 6. A method of producing a metallic article according to claim 2 wherein the substrate contains niobium as a major constituent.

0.7 Al O =7.3 percent SiO References Cited UNITED STATES PATENTS ALFRED L. LEAVI'IT, Primary Examiner.

I. R. BATTEN, IR., Assistant Examiner.

US. Cl. X.R.

l??? UNm-m Sui/mas ln' ricw owmc C l) H'lll F 1C A. l: O l? C O R H. E Cl i O- N 13234.21. ..4-l. o- Dated 611. 1969 It is certified that error appears in the 3b0ve-id0ntified patent and that said LeLu-rs Patent are hereby corrected as shown below:

Column 2, lines 35 and 40, the equals sign should be a hyphen MI in both cases. Column 3, line 22, "mitxure" should be--mixture Column 3, line 27, "ilumina" should be--alumina--. Column 4, line 9, the equals sign should be a hyphen.

SIGNED AND SEALED MAR 101970 (SEAL) Attest:

E M.Flth

dwarf! m It WILLIAM E. soau m, .m.

Attestlng Officer Commissioner of Patents 

